The aqueous solutions described hereinbefore become available in the process described in European patent application No. 0,206,377. This known process concerns extraction of a transition metal from an organic medium by contacting it with an aqueous phase in the presence of a cyclic phosphite of the type described hereinbefore, so as to obtain an aqueous extract phase containing said metal. Such transition metals include all the noble and non-noble metals of Group VIII of the Periodic Table of the Elements. This process is very suitable when the organic medium contains rhodium or cobalt, in complex combination with a compound of trivalent phosphorus, which rhodium or cobalt has been used as a catalyst component for the preparation of aldehydes by hydroformylation of an ethylenically unsaturated compound with carbon monoxide and hydrogen in the presence of a solvent. The aldehydes are meant to include herein the corresponding alcohols which may have been formed by reduction of the aldehydes. Although the amounts of rhodium required for the hydroformylation are generally very small, rhodium and the compounds thereof are so expensive that the catalyst should be recovered completely or practically completely. Moreover, such a recovery method should be simple. Therefore, attempts have been made to recover rhodium from said aqueous solutions. The aqueous solution may be separated by means of distillation at sub-atmospheric pressure at a relatively low temperature into a distillate fraction containing water and a residual fraction containing rhodium in complex combination with the cyclic phosphite. The rhodium in complex combination with the cyclic phosphite present in the residual fraction may be reused for hydroformylation, but a disadvantage thereof is that the aldehydes formed usually do not have a sufficiently high linearity. The linearity is defined herein as the percentage of n-aldehydes in the mixture of aldehydes formed. It is therefore usually more attractive to convert the complex of rhodium with the cyclic phosphite into a hydroformylation catalyst comprising rhodium in complex combination with a phosphine or with a phosphite other than the said cyclic phosphite. According to European patent application No. 0,206,377, this may be carried out by burning the rhodium-cyclic phosphite complex present in said residual fraction with formation of rhodium oxide, which, in turn, is converted in a known manner into a hydroformylation catalyst. This known process for the recovery of rhodium is not very simple, distillation, burning and preparation of a hydroformylation catalyst starting from rhodium oxide being involved. Moreover, distillation of water requires much energy.
It is an object of the present invention to recover the transition metal in a very simple manner.
A further object is to recover the transition metal without having to distill off water.
Another object is to recover the transition metal completely or practically completely.
Still another object is to recover the transition metal in complex combination with any desired ligand.